Abstract The dispersion measure of fast radio bursts (FRBs) encodes the integrated electron density along the line of sight, which is typically dominated by the intergalactic medium contribution in the case of extragalactic FRBs. In this paper, we show that incorporating wide-field spectroscopic galaxy survey data in the foreground of localized FRBs can significantly improve constraints on the partition of diffuse cosmic baryons. Using mock dispersion measures and realistic light-cone galaxy catalogs derived from the Millennium simulation, we define spectroscopic surveys that can be carried out with 4 and 8 m class wide-field spectroscopic facilities. On these simulated surveys, we carry out Bayesian density reconstructions in order to estimate the foreground matter density field. In comparison with the “true” matter density field, we show that these can help reduce the uncertainties in the foreground structures by ∼2–3× compared to cosmic variance. We calculate the Fisher matrix to forecast that N = 30 (96) localized FRBs should be able to constrain the diffuse cosmic baryon fraction to ∼10% (∼5%) and parameters governing the size and baryon fraction of galaxy circumgalactic halos to within ∼20%–25% (∼8%–12%). From the Fisher analysis, we show that the foreground data increase the sensitivity of localized FRBs toward our parameters of interest by ∼25×. We briefly introduce FLIMFLAM, an ongoing galaxy redshift survey that aims to obtain foreground data on ∼30 localized FRB fields.